Large scale commercial exploitation of certain oil sands and shale oil resources, available in huge deposits in Alberta and Venezuela, has been impeded by a number of problems, especially cost of extraction and environmental impact. The United States has tremendous coal resources, but deep mining techniques are hazardous and leave a large percentage of the deposits in the earth. Strip mining of coal involves environmental damage or expensive reclamation. Oil shale is also plentiful in the United States, but the cost of useful fuel recovery has been generally noncompetitive. The same is true for tar sands, which occur in vast amounts in Western Canada, which due to their viscosity are often not cost competitive to produce.
Materials such as oil shale, tar sands, and coal are amenable to in situ heat processing to produce gases and hydrocarbonaceous liquids. Generally, the heat develops the porosity, permeability and/or mobility necessary for recovery. Oil shale is a sedimentary rock which, upon pyrolysis or distillation, yields a condensable liquid, referred to as a shale oil, and non-condensable gaseous hydrocarbons. The condensable liquid may be refined into products that resemble petroleum products. Oil sand is an erratic mixture of sand, water and bitumen with the bitumen typically present as a film around water-enveloped sand particles. Using various types of heat processing the bitumen can, with difficulty, be separated from the sands. Also, as is well known, coal gas and other useful products can be obtained from coal using heat processing.
In the destructive distillation of oil shale or other solid or semi-solid hydrocarbonaceous materials, the solid material is heated to an appropriate temperature and the emitted products are recovered. This appears a simple enough goal but, in practice, the limited efficiency of the process has prevented achievement of large scale commercial application. Substantial energy is needed to heat the shale, and the efficiency of the heating process and the need for relatively uniform and rapid heating have been limiting factors on success. In the case of tar sands, the volume of material to be handled, as compared to the amount of recovered product, is again relatively large, since bitumen typically constitutes only about ten percent of the total weight. Material handling of tar sands is particularly difficult even under the best of conditions, and the problems of waste disposal contribute to cost inefficiencies.
There have been a number of prior proposals set forth for the upgrading of useful fuels from oil shales and tar sands in situ but, for various reasons, none has gained commercial acceptance and widespread application. One category of such techniques utilizes partial combustion of the hydrocarbonaceous deposits, but these techniques have generally suffered one or more of the following disadvantages: lack of precise control of the combustion, environmental pollution resulting from disposing of combustion products, and general inefficiency resulting from undesired combustion and waste of the resource.
Another category of proposed in situ upgrading techniques would utilize electrical energy for the heating of the formations. For example, in U.S. Pat. No. 2,634,961 there is described a technique wherein electrical heating elements are imbedded in pipes and the pipes are then inserted in an array of boreholes in oil shale. The pipes are heated to a relatively high temperature and eventually the heat conducts through the oil shale to achieve a pyrolysis thereof. Since oil shale is not a good conductor of heat, this technique is problematic in that the pipes must be heated to a considerably higher temperature than the temperature required for pyrolysis in order to avoid inordinately long processing times. However, overheating of some of the oil shale is inefficient in that it wastes input electrical energy, and may undesirably carbonize organic matter and decompose the rock matrix, thereby limiting the yield.
Further electrical in situ techniques have been termed as “ohmic ground heating” or “electrothermic” processes wherein the electric conductivity of the formations is relied upon to carry an electric current as between electrodes placed in separated boreholes. An example of this type of technique, as applied to tar sands, is described in U.S. Pat. No. 3,848,671. A problem with this technique is that the formations under consideration are generally not sufficiently conductive to facilitate the establishment of efficient uniform heating currents.
Variations of the electrothermic techniques are known as “electrolinking”, “electrocarbonization”, and “electrogasification” (see, for example, U.S. Pat. No. 2,795,279). In electrolinking or electrocarbonization, electric heating is again achieved via the inherent conductivity of the fuel bed. The electric current is applied such that a thin narrow fracture path is formed between the electrodes. Along this fracture path, pyrolyzed carbon forms a more highly conducting link between the boreholes in which the electrodes are implanted. Current is then passed through this link to cause electrical heating of the surrounding formations. In the electrogasification process, electrical heating through the formations is performed simultaneously with a blast of air or steam.
Generally, the just described techniques are limited in that only relatively narrow filament-like heating paths are formed between the electrodes. Since the formations are usually not particularly good conductors of heat, generally only non-uniform heating is achieved. The process tends to be slow and requires temperatures near the heating link that are substantially higher than the desired pyrolyzing temperatures, with the attendant inefficiencies previously described.
Another approach to in situ upgrading has been termed “electrofracturing”. In one variation of this technique, described in U.S. Pat. No. 3,103,975, conduction through electrodes implanted in the formations is again utilized, the heating being intended, for example, to increase the size of fractures in a mineral bed. In another version, disclosed in U.S. Pat. No. 3,696,866, electricity is used to fracture a shale formation and a thin viscous molten fluid core is formed in the fracture. This core is then forced to flow out to the shale by injecting high pressured gas in one of the well bores in which an electrode is implanted, thereby establishing an open retorting channel.
Radio frequencies (RF) have been used in various industries for a number of years. Induction heating of certain RF absorbent materials has been shown to be an efficient heating method. The nature and suitability of RF heating depends on several factors. In general, most materials accept electromagnetic waves, but the degree to which RF heating occurs varies widely. RF heating is dependent on the frequency of the electromagnetic energy, intensity of the electromagnetic energy, proximity to the source of the electromagnetic energy, conductivity of the material to be heated, and whether the material to be heated is magnetic or non-magnetic. Pure hydrocarbon molecules are substantially nonconductive, of low dielectric loss factor and nearly zero magnetic moment.
RF absorbent materials, on the other hand, absorb RF readily and are heated. This increase in temperature can be attributed to two effects. Joule heating is due to ionic currents induced by the electric fields that are set up in the absorber. These ionic currents cause electrons to collide with molecules in the material and resistance heating results. The other effect is due to the interaction between polar molecules in the absorber and high frequency electric fields. The polar molecules begin to oscillate back and forth in an attempt to maintain proper alignment with the electric field. These oscillations are resisted by other forces and this vibratory resistance is converted into heat.
The RF part of the electromagnetic (EM) spectrum is generally defined as that part of the spectrum where electromagnetic waves have frequencies in the range of about 3 kilohertz (3 kHz) to 300 gigahertz (300 GHz). Microwaves are a specific category of radio waves that can be defined as radiofrequency energy where frequencies range from several hundred MHz to several GHz.
One common use of this type of energy is the household cooking appliance known as the microwave (MW) oven. Microwave radiation couples with, or is absorbed by, non-symmetrical molecules or those that possess a dipole moment, such as water. In cooking applications, the microwaves are absorbed by water present in food and microwaves typically use a frequency of about 2.4 GHz for heating water. Free water vapor molecules, in contrast asborb in the 22 GHz range. Once the water absorbs the energy, the water molecules rotate and generate heat. The remainder of the food is then heated through a conductive heating process from the heated water molecules.
In general, the above described techniques are limited by the relatively low thermal and electrical conductivity of the bulk formations of interest. While individual conductive paths through the formations can be established, heat does not radiate at useful rates from these paths, and efficient heating of the overall bulk is difficult to achieve.
RF has been used for downhole upgrading, see e.g., US20060180304. However, in US20060180304 the EM energy is used to directly heat the oil components once the connate water has evaporated off. With direct heating of oil, it is said to be possible to control the temperature and avoid overheating carbonization effects.
US20100294489 by some of the same inventors as the instant invention, is similar to the work described herein. However, that work employs microwaves in the Ghz range, not radio waves, and thus has higher energy requirements than described herein.
Thus, what is needed in the art are more cost effective methods of using RF energies to produce heavy oils.